1. Field of the Invention
The present invention relates to an ion source section of a mass spectrometer for ionizing a gaseous sample introduced therein.
2. Description of the Prior Art
A gas chromatograph (GC) is provided for separating a gaseous mixture into its various constituent parts. A mass spectrometer (MS) is provided for analyzing a gaseous sample to determine its molecular structure. If the output of a gas chromatograph is coupled to the input of a mass spectrometer, the combined instrument is known as a gas chromatograph/mass spectrometer (GC/MS).
FIG. 3, for example shows a conventional ion source section of a mass spectrometer coupled to a gas chromatograph. The ion source section comprises an ionization chamber 1, an ion extraction electrode 2, an ion lens system 3, and a quadrupole electrode 4 for a quadrupole mass spectrometer. A sample gas is introduced into the ionization chamber 1, vertically to the plane of the drawing, through a sample introduction inlet 1b. A filament 1f is provided outside the ionization chamber 1 for injecting electrons into the ionization chamber 1 through an electron injection aperture 1e so that the sample molecule is ionized within the ionization chamber 1. An electron trap 1t is provided for capturing electrons passing through the ionization chamber 1.
FIG. 4 is a rear side of the apparatus of FIG. 3, showing a heater 5 embedded within the ionization chamber 1 for maintaining the sample gas introduced into the chamber 1 at a predetermined temperature. A column 6 for the gas chromatograph is coupled to the ionization chamber 1 through an interface block 7. The interface block 7 is a metal block attached to an outer housing 8 of the mass spectrometer. An introduction aperture 7h coupled to the column 6 of the gas chromatograph passes through the interface block 7. A heater 9 is embedded within the interface block 7. A temperature sensor 10 is provided for generating a control signal to keep the interface block 7 at a predetermined temperature. Because the ionization chamber 1 injects ion to the mass spectrometer portion, which is grounded, the chamber 1 should have a voltage different from the ground. Therefore, it is necessary to electrically isolate the ionization chamber 1 and the interface block 7 when introducing the gas. For this purpose, an isolator 11 having a throughhole is interposed between the interface block 7 and the ionization chamber 1.
The above-structured ion source section of the mass spectrometer has the following problems. Since the sample molecules are attached to the inside wall of the ionization chamber 1 so that the resultant mass spectra may make a ghost, it is necessary to clean the inside of the ionization chamber 1 some times. However, the ionization chamber 1 is secured to with the ion lens system 3 by a bolt 3b. So, in order to remove the ionization chamber 1, the ion lens system 3 must be dismantled and it is very troublesome to reassemble the ionization chamber after cleaning. It is preferred that the ionization chamber 1 and the interface block 7 be kept at the same temperature. The isolator 11 which is interposed between them, has a low thermal conductivity. It is, therefore, impossible to heat the ionization chamber 1 by the conducting heat from the interface block 7. A heater must be provided individually in both the interface block 7 and the ionization chamber 1, and therefore is rather difficult to continuously keep them at the same temperature. For this reason, further, the ionization chamber 1 is difficult to be disassembled and reassembled for cleaning.